The invention relates to an active vibration control system, and more particularly the invention relates to an active vibration control system for reducing fuselage vibration in a helicopter where the active control system actuators are optimally located in housings made integral with the exterior of the helicopter fuselage.
During helicopter operation, vibratory disturbances are transmitted from the rotor to the helicopter fuselage structure. The vibration is a source of irritation and discomfort to the passengers seated in the helicopter passenger compartment and as reflected in the related prior art, in an effort to improve passenger comfort and pilot performance, numerous approaches to controlling fuselage vibration in helicopter fuselages have been developed. Generally the proposed prior art approaches to limiting helicopter fuselage vibration can generally be divided into two separate technical classes: those that attempt to isolate the rotor vibratory disturbances from the fuselage and those that treat the vibration on the fuselage structure itself. Within the latter class, there are two approaches: those that relate to techniques for integrating passive tuned vibration absorbers (TVAs) with the fuselage, and those related to the use of active vibration control systems (AVCs) integrated with the fuselage structure. TVAs and AVCs control structural vibration in the fuselage structure and in the passenger compartment.
Specifically, TVAs are comprised of a low frequency, flexible suspended tuning mass that is tuned by adjusting either the stiffness of the mass"" associated flexible suspension or the actual mass of the TVA. The TVA mass is sized, shaped and suspended in the manner required to reduce the magnitude of the vibration in the fuselage. Effective performance of the TVA is limited to a narrow range of frequencies and therefore one or more additional TVAs would be needed if vibration reduction is required for one or more other frequency range(s).
Although TVAs are effective for reducing vibration in certain environments and applications, prior art TVAs are not the best suited and most effective means for limiting vibration in a helicopter fuselage for a number of reasons. First, by their design and method of operation, the performance of a TVA is directly proportional to its weight. Therefore, when the TVA is adapted for use in a helicopter or other aircraft, in order to effectively damp the vibratory disturbances, the TVA weight can become quite significant, for example the weight of a single TVA for use in a helicopter can exceed one hundred (100) pounds for certain applications. Second, prior art TVAs can only offer localized vibration reduction at and near the point of attachment between the tuning mass and fuselage. Therefore, if it is necessary to control vibration at a number of locations along the fuselage a number of heavy, passive vibration absorption units might be required. In a helicopter application where minimizing helicopter weight is critical, the additional significant weight associated with one or more TVAs makes TVAs an unappealing approach to limiting helicopter fuselage vibration and problems.
Active Vibration Control (AVC) systems consist of one or more actuators intelligently driven by an electronics unit connected to vibration sensors located throughout the aircraft. The actuators are driven or vibrated at one or more frequencies that are harmonics of the main rotor speed. AVC systems overcome the shortcomings of TVAs in at least two ways. First, performance of an AVC system is not a direct function of the actuator weight. Rather, the performance of the AVC system is a function of both the location and operation of the actuators. Thus, an AVC system typically weighs considerably less than a TVA designed to perform the same vibration control function. Second, an AVC system can be designed to globally control vibration throughout the fuselage. Multiple TVAs are often required to control fuselage vibration in the entire cabin. Thus an AVC system offers performance that is considerably more global than a TVA. An AVC system could have fewer actuators than a TVA system and still be able to control vibration in substantially the entire cabin. An AVC can control noise and vibration at multiple frequencies simultaneously whereas a TVA can only control vibration at a single frequency.
The performance of any AVC system is highly dependent on the location of the actuators in the helicopter. Therefore it is critical that the actuators be coupled to the fuselage where they are most likely to globally affect vibration and limit the effects of the vibratory disturbances on the helicopter fuselage. In the relevant prior art, little has been disclosed regarding optimal placement of the actuators on helicopters. Actuator placement critically influences the effectiveness of the AVC system in at least two ways. First, the global and local vibrations are strongly influenced by actuator placement. Second, actuator placement dictates the amount of force that the actuators must produce to limit the structural vibration, and the greater the required force, the larger and heavier the actuator mass must be to minimize the vibration.
The prior art illustrates active vibration control systems with sensors located in many different locations in the cabin. Frequently helicopter actuators are comprised of hydraulic actuators located in the struts between the rotor transmission pylon and the fuselage. Locating the actuators between the pylon and fuselage is feasible for new aircraft where the actuators can be designed into the struts during the overall aircraft design. However, the actuators cannot be easily retrofitted into existing helicopters. The prior art also discloses AVC systems that use hydraulic inertial actuators made integral with the fuselage along the interior cabin roof. Locating actuators in this manner provides acceptable vibration reduction however the forces required to achieve good performance are high resulting in larger than desired actuators.
The foregoing illustrates limitations known to exist in present passive and active helicopter vibration control systems and methods. Thus, it is apparent that it would be advantageous to provide a suitable active vibration control system for a helicopter that is easily retrofittable on existing helicopters and also provides acceptable reduction of fuselage vibration by applying low forces using actuators that do not add significant weight to the helicopter, and are located at optimal locations along the helicopter fuselage. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In light of the current state of the art, this is accomplished by the present invention which is an AVC system for helicopters with optimal actuator placement in housings made integral with the helicopter fuselage on opposite sides of the exterior helicopter fuselage.
The helicopter includes a fuselage that defines an interior cabin and a fuselage exterior. At least one housing is located along the exterior of the fuselage. The active vibration control system for limiting fuselage vibration includes sensor means for sensing the fuselage vibration; controller means in signal receiving relation with the sensor means; and actuator means located in the at least one housing. The actuator means is in signal receiving relation with the controller means.
The housing locations are optimal with respect to maximizing performance, minimizing force requirements and minimizing installation difficulty. These optimal locations were discovered through extensive vibration testing and analysis. By locating the housings along the fuselage exterior, the housings are readily accessible to a technician installing or repairing the actuator or system.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.